GB2290919A

GB2290919A - A filling for loudspeakers and loudspeaker stands

Info

Publication number: GB2290919A
Application number: GB9511736A
Authority: GB
Inventors: Jonathan Gregory Michael White
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-06-21
Filing date: 1995-06-09
Publication date: 1996-01-10
Anticipated expiration: 2015-06-09
Also published as: GB9511736D0; GB9412427D0; GB2290919B

Abstract

Within the enclosure is disposed a body of a medium 28 having a multiplicity of interconnected air chambers. The medium may be formed from a multiplicity of convolute polystyrene pieces 30 bound into a body by adhesive. Also disclosed is a loudspeaker stand 40 having tubular legs 46 each filled with a different quantity of granular material 50 such that the resonant frequencies of the legs do not coincide with each other. <IMAGE>

Description

LOUDSPEAKERS The present invention relates to loudspeakers and in particular to cabinet loudspeakers comprising an enclosure which encloses a volume of air and one or more drive units mounted at an.aperture in a wall of the enclosure the drive unit being arranged to radiate sound energy to air within and outside of the enclosure.

One of the most major problems which occurs in the design of loudspeakers, and most especially in the design of loudspeakers for hi-fidelity audio reproduction, is in arranging for the output of the loudspeaker to be, as far as possible, uniform across the range of frequencies over which it is intended to operate. A particular difficulty arises in ensuring that the loudspeaker has adequate response at the lower part of its range and does not "fall off when reproducing bass notes. This problem is particularly acute when it is desired to minimise the size of the loudspeaker enclosure.

Perhaps the most common arrangement for minimising this problem is the use of a base reflex enclosure. This type of enclosure has a port comprising a tubular passageway which extends from within the enclosure and is open to the atmosphere, typically, close to a drive unit of the loudspeaker. This arrangement provides a boost in frequency response at the resonant frequency of the port itself, of the air within the enclosure, and of the drive unit. These three resonant frequencies are chosen, during design of the loudspeaker, to provide as even as possible a distribution of frequencies during sound reproduction. However, since these resonances occur at discreet peaks, the results will clearly be compromised.

An alternative design of loudspeaker incorporates what is known as a "transmission line" enclosure. In this type of loudspeaker, the enclosure is divided into a plurality of chambers, interconnected to provide an air passageway between a drive unit and the air outside the enclosure. Each chamber has its own resonant frequency at which the output from the loudspeaker is boosted. By providing several (typically three or four) chambers, the output of the loudspeaker can be boosted over a range of frequencies. However, this arrangement also produces a number of output peaks, which limits the ability of the loudspeaker to generate the desired uniform output response.

It is known that the presence of angular corners within a loudspeaker enclosure can contribute to degradation of sound quality, and loudspeakers have been produced with rounded corners. However, the cost of producing such loudspeakers is prohibitably great for all but highly expensive loudspeakers.

It is an aim of the present invention to provide a loudspeaker which has favourable output characteristics, particularly, but not exclusively, in the lower part of the audible spectrum.

According to a first aspect of the invention, there is provided a cabinet loudspeaker in which the enclosure contains a medium within which is defined a multiplicity of interconnected air chambers.

Each air chamber within the medium generates a resonance at its natural frequency, the provision of a large number of such chambers causing these resonances to be distributed over a wide range of frequencies.

Preferably, the medium substantially fills the enclosure.

The medium may comprise a multiplicity of discrete pieces, the air chambers being constituted by spaces between the pieces. This provides a convenient and economical construction for the medium. Preferably, the pieces are bonded together to resist vibration of the pieces. Conveniently, the pieces may be made of expanded polystyrene and bonded together with adhesive.

In one embodiment, the enclosure is sealed. That is to say it has no reflex port or other free air communication between the inside and outside of the enclosure.

In an alternative embodiment, a reflex port is provided. Preferably the reflex port is directed forwardly towards a listener. Within the chamber, the port is beneficially totally surrounded by medium.

The loudspeaker preferably comprises at least two drive units, a first being for the reproduction of low frequencies and a second for the reproduction of high frequencies. Preferably, a cross-over is provided to prevent low frequency signals being supplied to the drive unit intended for high frequencies.

It has also been noticed by the applicant that loudspeaker stands of conventional, tubular steel construction have a tendency to resonate at audible frequencies, with the result that they can create an unwelcome boost to particular frequencies generated by a loudspeaker mounted thereon. Loudspeaker stands are known which are of particularly robust construction or which are made of special materials, for example, stone, which have no appreciable resonance. However, these stands tend to be difficult and expensive to produce. It is a further aim of the invention to provide loudspeaker stands which solve or at least ameliorate this problem, particularly, but not exclusively, for use with the loudspeakers of the first aspect of the invention.

According to a second aspect of the invention, there is provided a stand for a loudspeaker comprising a plurality of tubular supporting legs, at least two and preferably all legs being partially filled with solid or liquid material, the amount of material in each of these legs being selected such that the two legs have a resonant frequency different from one another and from any other leg of the loudspeaker stand.

By this arrangement, the resonant effects of the loudspeaker stand can be minimised.

The legs may conveniently be filled with a granular solid material, such as sand.

From a third of its aspects, the present invention provides, in combination, a loudspeaker according to its first aspect and a loudspeaker according to its second aspect, in which the resonant frequency of at least one of the legs of the stand is selected to coincide with frequencies at which the output of the loudspeaker is deficient.

Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a view of a loudspeaker being a first embodiment of the invention, part cutaway, and a loudspeaker stand embodying the present invention; Figure 2 is a circuit diagram of a cross-over circuit for use in a loudspeaker of the present invention; Figure 3 shows a piece suitable for construction of a medium for use in the loudspeaker of Figure 1; Figure 4 is graph showing the output of the loudspeaker over a range of frequencies; Figure 5 is a general perspective view of a loudspeaker being a second embodiment of the invention; and Figure 6 is a sectional view of an upper portion of the loudspeaker of Figure 5.

With reference to the drawings, a loudspeaker comprises an enclosure 10 having rectangular top, bottom and side panels 12,14,16 and a front panel 18, and being approximately 290 mm x 190 mm x 290 mm in, respectively, height, width and depth.

Base and treble drive units 20,22 are mounted through apertures in the front panel 18, such that an air-driving cone 24 of the base drive unit 20, on application of an alternating electrical signal to the base drive unit 20 radiates sound energy outwards from the loudspeaker and into the enclosure 10 of the loudspeaker. The enclosure and drive unit assembly is substantially air-tight although a small amount of leakage is inevitable and is, indeed, welcome. This leakage permits the passage of a small amount of air into and out of the loudspeaker to avoid the need to provide venting to compensate for temperature changes and changes in barometric pressure.

Within the enclosure, there is disposed a body of a medium 28 arranged to substantially fill the interior of the enclosure, apart from that space required to accommodate the drive units 20,22. The medium 28 comprises a multiplicity of expanded polystyrene pieces 30, each substantially as illustrated in Figure 3. Each piece 30 is formed with an approximately rectangular cross-section curved upon itself into an S-shape. The precise volume and disposition of the medium 28 may vary in accordance with the particular design of loudspeaker cabinet. However, this can be determined by routine experiment in a manner well-known to those skilled in the art.

The medium is formed by placing into a mould of substantially the same size and shape of the enclosure 10 sufficient pieces 30 to form a body of medium 28 of size suitable for filling the enclosure. A quantity of thin liquid adhesive, such as woodworking or PVA adhesive thinned with water, is then poured over the pieces. If necessary, this may be repeated several times until the pieces 30 appear to be bound together into one mass. The body of medium 28 is then placed into the enclosure 10 of a loudspeaker, following which the remaining components of the loudspeaker are assembled.

It will be appreciated that the medium 28 is permeated with a multiplicity of air chambers formed by the folds of the pieces 30, sealed together with adhesive. The chambers formed are randomly distributed and are of various sizes, one chamber being formed by the folds of one, two or more pieces, interconnection by adhesive. These chambers together act in a manner similar to a transmission line cabinet with many tens or hundreds of chambers.

It is believed that the medium 28 enhances loudspeaker output through its being highly refractive to sound energy of mid-range frequencies. Traditionally, speakers are often provided internally with a damping medium. This has the undesirable effect of slowing movement of the cone of the base drive unit, which results in reduction of emphasis on bass transients with a perceived loss of bass "slam" when reproducing, for example, bass drum beats. Removing the damping from a traditional cabinet, may improve bass slam, but has the disadvantage of causing mid-range frequencies to be reflected internally, with undesirable results.

Additionally, the presence of the medium effectively "hides" the angular corners of the interior of a conventional cuboidal box loudspeaker.

The improvement appears to be due to the air being unable to escape from the chambers at higher frequencies, so causing different amounts of air to be trapped within the medium 28 at higher frequencies. Considering the air behind the drive unit cone, it will be seen that movement of this air into and out of the medium 28 becomes more restricted frequency increases, effectively making the air stiffer to oscillation at high frequencies. This stiffening behaviour is similar to that of the surround of the loudspeaker cone. This is effective to maximise the energy transfer between the interior of the enclosure and the cone, resulting in good output characteristics over a wide range of frequencies.

With reference to Figure 4, there can be seen the output spectrum of the loudspeaker. The horizontal axis is the logarithm of frequency in Hz, while the vertical axis represents the output of the loudspeaker, the scale being in dB. It will be seen from Figure 4 that the output of the loudspeaker shows that low frequency response is considerably improved over those speakers typical in the prior art. For example, it can be seen that the -3 dB point in the response occurs as low as 26 Hz, as compared with 100 Hz typical in the prior art.

The contribution made by the medium 28 to the bass response occurs mainly in the frequency range marked A.

Each drive unit 20,22 is fed from a respective pair of terminals on a rear panel of the enclosure 10. The treble drive unit 22 is connected as shown in Figure 2.

This ensures that the signals fed to the treble drive unit are, as far as possible, in phase with those fed to the base drive unit 20, which may or may not be subject to filtration. Provision of a pair of terminals for each drive unit facilitates bi-wiring, allowing signals to the different drive units to be carried from the amplifier to the loudspeaker along separate conductors.

The loudspeaker may be supported on a stand 40, being an embodiment of the second aspect of the invention, and illustrated in Figure 1. The stand 40 comprises a top plate 42 from which projects upwardly a plurality of spikes 44 on which the loudspeaker is supported. The top plate 42 is carried on a plurality of (in this case four) vertical legs 46, each being of tubular metal. The legs 46 are braced to one another by connecting members 48 close to their respective lower ends. Thus far, the stand 40 is of substantially conventional construction.

A lower portion of each leg 46 is filled with sand or a similar solid, granular material, as indicated at 50.

It will be found that the resonant frequency of each leg 46 changes in dependence upon the amount of sand contained therein. The legs 46 are filled to mutually differing heights such that each leg 46 of the stand has a different resonant frequency, so ensuring that the stand 40 does not act to artificially boost the output of the loudspeaker at a particular frequency. Each leg 46 may have its resonant frequency set to correspond to a frequency at which the output from the loudspeaker is lower than would be desired, but benefit may be gained even if this is not the case.

With reference to Figure 5 of the drawings, an alternative embodiment of a cabinet loudspeaker according to the invention is shown. This embodiment is a floorstanding design, but the principles of its construction could equally be applied to a stand-mounted loudspeaker.

The loudspeaker comprises an upright cabinet 100 of rectangular cross-section, its dimensions being approximately 190mm in width, 270mm in depth and 900mm in height. The cabinet is divided horizontally into five chambers 102 .. 110.

The first chamber 102 is uppermost, and constitutes an enclosure for a high-frequency drive unit 112., the drive unit being mounted to radiate sound energy in an aperture in a front wall of the enclosure. The internal volume of the first chamber is approximately 2R. The second chamber 104, located immediately below the first chamber 102, constitutes an enclosure for a low frequency drive unit 114. The internal volume of the first chamber is approximately 5.6R. The third chamber 106 is located immediately below the second chamber 104. A forward opening reflex port 116, approximately 150mm long, is provided in the third chamber 106, to exit therefrom directed towards a listener. No drive unit is located in the third chamber 106. The internal volume of the third chamber 106 is approximately 3.5R.

The first, second and third chambers 102, 104, 106 are separated by an internal baffle, there being a relatively small elliptical hole in each baffle o provide an air passage between adjacent chambers 102, 104, 106.

Each of these chambers is filled with medium, as described above.

The three-chambered arrangement described above could, on its own constitute a loudspeaker, particularly for use on a stand. However, in the illustrated embodiment, the cabinet is extended downwards to form a floor-standing unit.

Immediately below the third chamber 106, and separated from it by a closed baffle, is a fourth chamber 108. In this chamber may be housed electronic circuits necessary for operation of the loudspeaker. This arrangement is advantageous because such circuits are relatively isolated from vibration (unlike the case where they are mounted in a chamber that contains a drive unit) and so undesirable microphonic effects are minimised.

Finally, in a lowermost chamber 110, there is contained ballast, such as lead shot, to enhance the stability of the loudspeaker cabinet.

Figure 6 illustrates a modification which could be made to either of the above embodiments. In this arrangement, two drive units 124, 126 share a chamber 120. However, they are partially separated from one another by a divider 122 which extends, typically 100 to 150mm, from a wall 130 on which the drive units 124, 126 are mounted into the chamber 120. The divider 122 serves to prevent propagation of sound waves linearly from one drive unit to another, with a consequent improvement in sound quality in some circumstances. The chamber 120 is completely filled by the medium, as described above, and as shown at 128 in Figure 6.

Claims

1. A cabinet loudspeaker having an enclosure for a drive unit which enclosure contains a medium within which is defined a multiplicity of interconnected air chambers.

2. A cabinet loudspeaker according to claim 1 in which the medium substantially fills the enclosure.

3. A cabinet loudspeaker according to claim 1 or claim 2 in which the medium comprises a multiplicity of discrete pieces, the air chambers being constituted by spaces between the pieces.

4. A cabinet loudspeaker according to claim 3 in which the pieces are bonded together to resist vibration of the pieces.

5. A cabinet loudspeaker according to claim 3 or claim 4 in which the pieces are made of expanded polystyrene and bonded together with adhesive.

6. A cabinet loudspeaker according to any preceding claim in which the enclosure is sealed.

7. A cabinet loudspeaker according to any one of claims 1 to 5 in which the enclosure has a port through which air may pass into and out of the enclosure.

8. A cabinet loudspeaker according to any preceding claim comprising at least two drive units.

9. A cabinet loudspeaker according to claim 8 comprising at least two drive units, a first being for the reproduction of low frequencies and a second for the reproduction of high frequencies.

10. A cabinet loudspeaker according to claim 8 or claim 9 in which at least two drive units are contained within the enclosure, and there is a shelf-like projection extending from the wall of the enclosure between the drive units.

11. A cabinet loudspeaker according to any one of claims 8 to 10 comprising a cross-over circuit to prevent low frequency signals being supplied to a drive unit intended for high frequency reproduction.

12. A cabinet loudspeaker having first and second enclosures at least one being for a drive unit the enclosures containing a medium within which is defined a multiplicity of interconnected air chambers, and there being an air passage interconnecting the enclosures.

13. A cabinet loudspeaker according to claim 12 in which both first and second enclosures are for a respective drive unit.

14. A cabinet loudspeaker according to claim 12 or claim 13 in which at least one of the first enclosure, the second enclosure or a further enclosure contains a reflex port to provide an air path from within to outside of the cabinet.

15. A cabinet loudspeaker according to any preceding claim comprising a chamber having a wall on which first and second drive units are mounted, there being a divider projecting from the wall part way across the chamber between the drive units.

16. A cabinet loudspeaker according to any preceding claim in which the air chambers are distributed randomly.

17. A cabinet loudspeaker substantially as herein described with reference to Figures 1 to 3 of the accompanying drawings.

18. A cabinet loudspeaker substantially as herein described with reference to Figures 1 to 3 and 5 of the accompanying drawings.

19. A cabinet loudspeaker according to claim 17 or claim 18 as modified by Figure 6 of the accompanying drawings.

20. A stand for a loudspeaker comprising a plurality of tubular supporting legs, at least two of which legs being partially filled with solid or liquid material, the amount of material in each of these legs being selected such that the two legs have a resonant frequency different from one another and from any other leg of the loudspeaker stand.

21. A stand for a loudspeaker according to claim 20 in which the legs are filled with a granular solid material.

22. A stand for a loudspeaker according to claim 21 in which the granular solid material is sand.

23. A stand for a loudspeaker substantially as herein described with reference to Figure 1 of the accompanying drawings.

24. In combination, a cabinet loudspeaker according to any one of claims 1 to 19 and a stand according to any one of claims 20 to 23, in which the resonant frequency of at least one of the legs of the stand is selected to coincide with frequencies at which the output of the loudspeaker is deficient.